‘Umar (may Allaah be pleased with him) drew away the leather covers which were covering the spoils from the Battle of Jalula in front of the people. Upon seeing the rubies, aquamarines and other assorted jewels, he started weeping. ‘Abdur Rahmaan bin ‘Awf asked, “What makes you weep O leader of the believers? By Allaah, it is a time to thank Him (not to feel sorrow).”
‘Umar said, “By Allaah! What makes me weep? By Allaah, a people are not given such things but they start becoming jealous and hating each other. And they do not become jealous of each other, except that they start fighting amongst themselves.”
[Taareekh At-Tabaaree 4/480]